1. Field of the Invention
The present invention relates to an apparatus for controlling the temperature of a heat roller used in an electrophotographic copying machine or the like for thermally fixing toner on copying paper, and the invention particularly relates to processing of signals for a pyroelectric infrared thermal sensor used for the measurement of the heat roller temperature.
2. Prior Art
In electrophotographic copying machines and the like, a heat roller is used for fixing the image of toner formed on copying paper by exerting heat and pressure on the paper.
In this case, if the temperature of the heat roller surface is too low, a faulty fixing of toner will result, while if the roller surface temperature is too high, a faulty offset of toner or winding of copying paper will result. On this account, the heat roller must have its surface temperature controlled within a certain range.
For this purpose, there has been practiced feedback control of the heat roller temperature in which the roller surface temperature is brought to a predetermined temperature by turning on and off a power supply in accordance with the detected roller surface temperature.
There is known a technique for measuring the heat roller surface temperature using a pyroelectric infrared thermal sensor (will be termed simply "pyroelectric sensor" hereinafter).
The pyroelectric sensor operates to detect in the form of a voltage signal or the like the temperature variation of a heat source object in response to a change in the spontaneous polarization of a pyroelectric member caused by the incident infrared rays from the heat source object under measurement. The sensor voltage output is theoretically given as follows. EQU V=K.sub.1 W, W=K.sub.2 T,
Accordingly, EQU V=K.sub.1 K.sub.2 T, (1)
Where V denotes the sensor output voltage, W denotes the infrared radiation energy emitted from an object, T denotes the absolute temperature of the object, and K.sub.1 and K.sub.2 are constants.
The pyroelectric sensor is operative to detect a somewhat sharp temperature variation (a temperature change caused by the incident infrared rays). Accordingly, it provides a zero output against a constant incident of the infrared rays. In a practical use, a chopper mechanism is employed to interrupt the incident infrared rays, thereby creating a sharp temperature variation. As will be appreciated from the above statement, the output of the pyroelectric sensor represents a relative temperature (or temperature difference) between the temperature of a heat source object under measurement and the chopper mechanism periphery temperature, or more preferably the pyroelectric sensor periphery temperature, (will be termed "chopper temperature" hereinafter). The sensor output voltage is theoretically given as follows. EQU V=.alpha.(T.sub.1.sup.4 -T.sub.0.sup.4), (2)
where .alpha. denotes a proportional constant, T.sub.1 denotes the temperature of the object under measurement, and T.sub.0 denotes the chopper temperature.
Accordingly, in order to know the heat roller surface temperature, the chopper temperature is measured and it is added to the relative temperature detected by the pyroelectric sensor.
FIG. 8 is a graph showing the result of heat roller temperature control, on which are plotted the actual temperature of the heat roller surface (solid line) and the apparent temperature of the heat roller surface (dashed line) obtained by summing the output of the pyroelectric sensor and the reference temperature (chopper temperature) measuring diode sensor (both outputs are also shown separtely). The actual temperature was measured by independent means.
As expressed by the equation (2), the output of the pyroelectric sensor is proportional to the fourth power of the heat roller surface temperature subtracted by the fourth power of the chopper temperature. Namely, the pyroelectric sensor output is not linearly proportional to the difference of temperature of the heat roller surface and in the chopper periphery, and the conflict is particularly significant in the region of larger temperature difference. For example, as shown in FIG. 8, when the chopper temperature is relatively low as is the case immediately after the copying machine has been turned on, the heat roller surface temperature detected as a composite signal derived from the pyroelectric sensor output and diode sensor output is higher than the actual temperature. Therefore, the power switching control for the heater based on the composite signal will result in a heat roller surface temperature lower than the predetermined temperature, and a faulty fixing of toner will result.